In Eqn. 2 , sap velocity is estimated from four types of
quantities: a ratio of temperature increases between two probes
(δ j/δ i), the thermal
diffusivity k , the positions of the two probes
(x i, x j,y i, y j), and the time
elapsed since the heat pulse length (t ). If the two probes are
parallel to one another (i.e., at identical azimuthal positions relative
to the flow axis), then y i =y j, giving